1. Field of the Invention
The present invention relates to a semiconductor position sensitive detector (PSD) for detecting the position of incident light.
2. Related Background Art
A semiconductor position sensitive detector (PSD) is known as a device for measuring the distance to an object (to be measured) by using the so-called triangulation principle. The PSD is mounted as an active distance measuring device in an image sensor such as a camera. In such an image sensor, the photographing lens is focused on the basis of the distance to the object which is measured by the PSD.
In the above PSD, the position of an incident light spot on the surface of the PSD moves in accordance with the distance to the object. The resistance value of the resistive layer of the PSD is divided in accordance with the incident light spot position, and an output current from the PSD changes in accordance with the resistance division ratio. The distance to the object can therefore be obtained on the basis of the output current. In distance measurement using the triangulation principle, when the distance to an object at a short distance changes, the position of an incident light spot greatly moves on the surface. In contrast to this, when the distance to an object at a long distance changes, the position of an incident light spot does not move much. That is, conventionally, the distance detection precision for an object at a long distance is lower than that for an object at a short distance. Under the circumstances, a PSD is disclosed in Japanese Patent Laid-Open No. 4-240511, in which the width of a resistive layer to be irradiated with an incident light is decreased linearly from the short distance side to the long distance side of the surface, so that the resistance division ratio of the resistive layer greatly changes even if the movement amount of an incident light spot from an object at a long distance is small.
In the PSD disclosed in the reference described above, the width of the resistive layer is increased linearly from the long distance side to the short distance side, i.e., the width of the resistive layer is decreased linearly from the short distance side to the long distance side. The resistive layer can be regarded as a set of minute resistors connected in the form of a matrix. Charges that are generated when light is incident on the resistive layer are divided on the basis of the resistance ratios between the incident light position and the electrodes on the two ends of the resistive layer. When only some of the group of minute resistors arrayed in the widthwise direction of the resistive layer are irradiated with incident light in the form of a spot, generated charges do not uniformly pass through the resistive layer along the lengthwise direction. Therefore, an expression representing the relationship between an incident light position and an output current, logically calculated from the shape of the resistive layer, varies depending on the incident light position and incident light shape. This makes it difficult to accurately compute an incident light position from an output current by using the single expression. That is, a plurality of different arithmetic circuits are required for different incident light positions and incident light shapes to obtain accurate incident light positions from output currents. In other words, in the above conventional PSD, only when the whole group of minute resistors arrayed in the widthwise direction of the resistive layer is irradiated with incident light, i.e., only when incident light in the form of a slit strikes across the resistive layer, an incident light position can be obtained by using a single arithmetic circuit.
The present invention has been made to solve the above problems, and has as its object to provide a semiconductor position sensitive detector which can further improve position detection precision as compared with the prior art, and is free from any limitations on incident light shapes.
A semiconductor position sensitive detector according to the present invention is characterized by comprising a resistive region lined up in a predetermined direction, and a plurality of conductive strips running from the resistive region such that different output currents are obtained from two ends of the resistive region in accordance with incident light positions on the surface, wherein the resistive regions have substantially the same resistivity, and gradually increase in width in a direction perpendicular to the predetermined direction from one end to the other end of the resistive region. Although the plurality of resistive regions are preferably continuous, respectively, with conductive strips being interposed therebetween, the resistive regions may be continuous, respectively, in contact with each other.
The position of incident light moves on the surface in accordance with the distance to the object. Charges generated in accordance with irradiation of the incident light flow into the resistive region through the conductive strips. Since the conductive strips run such that different output currents are obtained from the two ends of the resistive region in accordance with the incident light position on the surface, the incident light position can be obtained from these output currents.
A plurality of resistive regions gradually increase in width from one end to the other end and are substantially equal in resistivity. Since the narrow resistive regions have high resistances, output currents from the two ends of the resistive region greatly change even in a case wherein the incident light position only slightly moves on the surface with a change in the distance to the object.
Note that since light beam is incident in the part of the conductive strips, and generated charges are resistance-divided in the resistive region, the width of the resistive region can be reduced, and a desired resistance can be obtained even if the impurity concentration is increased to decrease the resistivity. That is, since the ratio of the minimum controllable impurity concentration to the total impurity concentration decreases with an increase in impurity concentration, variations in resistivity reduce, and the position detection precision improves.
In addition, the width of each resistive region is preferably a liner or quadratic function of a position from one end of the resistive region along a predetermined direction. Since the surface formed on each branch conductive layer is irradiated with incident light, the distance to the object can be computed from output currents from the two ends of the resistive region by using a function for distance detection which is derived from the fact that the width of each resistive region is a linear or quadratic function of a position regardless of the shape of incident light.
Assume that signal extraction electrodes for extraction of output currents are formed on the two ends of the resistive region. In this case, if the conductive strips adjacent to these electrodes are irradiated with incident light beams, since the signal extraction electrodes are irradiated with part of the incident light beams, the barycentric position of each incident light beam deviates from the true position toward the branch conductive layer side, resulting in a decrease in position detection precision.
The semiconductor position sensitive detector of the present invention is characterized by further comprising a semiconductor region which is adjacent to a predetermined one of the conductive strips, which runs from one end portion of the resistive region and having a smallest width, and has a resistivity lower than that of the resistive region, and a signal extraction electrode which is formed at a position into which a charge passing through the semiconductor region can flow without the mediacy of the resistive region in accordance with the incident light, and from which one of the output currents is extracted.
In the absence of the semiconductor region, if the branch conductive layer and signal extraction electrode which run from the narrowest resistive region are irradiated with incident light, an output current from the signal extraction electrode is reduced because the incident light is blocked by the signal extraction electrode. The semiconductor position sensitive detector of the present invention, however, comprises a semiconductor region, and hence charges generated in accordance with incident light entering the semiconductor layer flow into the signal extraction electrode without the mediacy of the resistive region to increase an output current from the signal extraction electrode. This makes it possible to bring the computed barycentric position of the incident close to the true position and improve the position detection precision.
When the resistive region is irradiated with light, the computed incident light position may deviate from the true value depending on the shape of the light. When, therefore, a higher precision is required, this semiconductor position sensitive detector comprises a light-shielding film formed on the resistive region, thereby further improving the position detection precision.
Furthermore, this semiconductor position sensitive detector is characterized in that the detector comprises a pair of signal extraction electrodes from which output currents from the two ends of the resistive region are respectively extracted, and if the resistive region is located between the signal extraction electrodes, the light-shielding film is made of an insulating material and covers the resistive region between the signal extraction electrodes. When the light-shielding film is made of an insulating film, even if the entire region of the resistive region between the signal electrodes is covered with the light-shielding film, the signal extraction electrodes are not short-circuited.
In addition, this light-shielding film is preferably made of a black photoresist. A light-shielding film can therefore be formed by developing and curing.